Fibroma of tendon sheath is defined by a USP6 gene fusion-morphologic and molecular reappraisal of the entity

Pathology of intra-articular tumours and tumour-like lesions: pearls, pitfalls and rarities from a general surgical pathology practice

Intra-articular tumours are uncommonly encountered in routine practice and may present diagnostic challenges to pathologists. Challenges unique to this site include distinction from more common reactive synovial conditions, which are far more common; histologic variability; superimposed reactive changes; and often, lack of provided clinicoradiological context. This article reviews the pathology of the synovial tumours and tumour-like lesions, including diagnostic pearls, pitfalls and rare entities.

Palmer Nodular Fasciitis Harboring a Novel SREBF1::USP6 Fusion Gene

The diagnosis of low-grade fibroblastic/myofibroblastic tumors of acral sites can be challenging. These tumors encompass a diverse group of neoplasms with a spectrum of biologic potential ranges from benign to overtly malignant. They often demonstrate significant clinical, radiologic, and immunophenotypic overlap, in which the molecular phenotype may play an important diagnostic role to arrive at the final diagnosis. Herein, we report a case of soft tissue mass lesion presented on the palm of an adult patient for four months. Histologically, the tumor consisted of primarily low-grade spindle cells expressing smooth muscle actin. Molecular testing revealed a novel SREBF1::USP6 fusion gene, confirming the final diagnosis of nodular fasciitis and ultimately expanding its molecular profile. This case highlights the diagnostic value of single, cost-effective, targeted molecular panel to arrive at an accurate diagnosis and provide helpful therapeutic information.

Unravelling the USP6 gene: an update

Ubiquitin-specific protease 6 (USP6) rearrangements have been identified in aneurysmal bone cyst, nodular fasciitis, myositis ossificans, fibro-osseous pseudotumour of digits and cellular fibroma of tendon sheath. These entities show clinical as well as histological overlap, suggesting they are all clonal neoplastic belonging to the same biological spectrum and referred to as 'USP6-associated neoplasms'. They all show a characteristic gene fusion formed by juxtaposition of the USP6 coding sequences to the promoter regions of several partner genes, leading to USP6 transcriptional upregulation.

Multimodality imaging features of USP6-associated neoplasms

Since the discovery of USP6 gene rearrangements in aneurysmal bone cysts nearly 20 years ago, we have come to recognize that there is a family of USP6-driven mesenchymal neoplasms with overlapping clinical, morphologic, and imaging features. This family of neoplasms now includes myositis ossificans, aneurysmal bone cyst, nodular fasciitis, fibroma of tendon sheath, fibro-osseous pseudotumor of digits, and their associated variants. While generally benign and in many cases self-limiting, these lesions may undergo rapid growth, and be confused with malignant bone and soft tissue lesions, both clinically and on imaging. The purpose of this article is to review the imaging characteristics of the spectrum of USP6-driven neoplasms, highlight key features that allow distinction from malignant bone or soft tissue lesions, and discuss the role of imaging and molecular analysis in diagnosis.

Recurrent FOSL1 rearrangements in desmoplastic fibroblastoma

The FOS gene family has been implicated in tumourigenesis across several tumour types, particularly mesenchymal tumours. The rare fibrous tumour desmoplastic fibroblastoma is characterised by overexpression of FOSL1. However, previous studies using cytogenetic and molecular techniques did not identify an underlying somatic change involving the FOSL1 gene to explain this finding. Prompted by an unusual index case, we report the discovery of a novel FOSL1 rearrangement in desmoplastic fibroblastoma using whole-genome and targeted RNA sequencing. We investigated 15 desmoplastic fibroblastomas and 15 fibromas of tendon sheath using immunohistochemistry, in situ hybridisation and targeted RNA sequencing. Rearrangements in FOSL1 and FOS were identified in 10/15 and 2/15 desmoplastic fibroblastomas respectively, which mirrors the pattern of FOS rearrangements observed in benign bone and vascular tumours. Fibroma of tendon sheath, which shares histological features with desmoplastic fibroblastoma, harboured USP6 rearrangements in 9/15 cases and did not demonstrate rearrangements in any of the four FOS genes. The overall concordance between FOSL1 immunohistochemistry and RNA sequencing results was 90%. These findings illustrate that FOSL1 and FOS rearrangements are a recurrent event in desmoplastic fibroblastoma, establishing this finding as a useful diagnostic adjunct and expanding the spectrum of tumours driven by FOS gene family alterations. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

USP6-associated soft tissue tumors with bone metaplasia: Clinicopathologic and genetic analysis and the identification of novel USP6 fusion partners

Introduction

Among those tumors with consistent USP6 rearrangement, some arise from soft tissue and show bone metaplasia, including myositis ossificans (MO), fibro-osseous pseudotumor of digits (FOPD), soft tissue aneurysmal bone cyst (ST-ABC) and fasciitis ossificans (FO). These lesions are easily confused with malignancies because they show a rapid growth rate and brisk mitoses. Here, we aim to clarify the clinicopathologic and genetic characteristics of this entity and analyze the correlations among the different subtypes in one of the largest cohorts.

Materials and Methods

The clinicopathologic features of 73 cases of MO, FOPD, ST-ABC and FO diagnosed at West China Hospital, Sichuan University from January 2010 to December 2021 were retrospectively analyzed. Forty-three undecalcified samples were analyzed by systematic genetic studies, including fluorescence in situ hybridization (FISH), reverse transcription polymerase chain reaction (RT-PCR), Sanger sequencing and next-generation-based sequencing were performed.

Results

This series included 40 males and 33 females aged 2 to 80 years old (median: 31 years). FOPD occurred in extremal soft tissue, while lower extremities (38/58, 65.5%) were the most commonly involved lesions in the other three subgroups. Histologically, proliferative myofibroblasts/fibroblasts with varying degrees of osteoid tissue were present. Fluorescence in situ hybridization (FISH) results indicated that 22 cases (22/27, 81.5%) were positive for USP6 rearrangement, and 5 cases were negative. Among those cases with positive FISH results, 18 underwent reverse transcription-polymerase chain reaction (RT-PCR) detection that successfully detected common USP6 fusion types. Thirteen cases showed COL1A1::USP6 fusion, one showed MYH9::USP6 fusion, and 4 were negative for common fusion types. Next-generation-based sequencing technology was performed on two lesions with negative RT-PCR results and novel fusion partners SNHG3 and UBE2G1 were discovered.

Conclusions

Our findings revealed that COL1A1 is the most common fusion partner in this entity, unlike primary aneurysmal bone cysts and nodular fasciitis. Notably, we believed that FO may demonstrate more similar clinicopathologic and genetic manifestations with MO/FOPD and ST-ABC instead of nodular fasciitis for involving lower limbs most frequently and showing recurrent COL1A1::USP6 fusion. Additionally, this study also found two novel USP6 fusion partners, which further expanded our knowledge of this neoplastic spectrum.

Infantile Fibrosarcoma and Other Spindle Cell Neoplasms of Infancy. A Review of Morphologically Overlapping yet Molecularly Distinctive Entities

BACKGROUND

Regardless of age at presentation, many soft tissue neoplasms have overlapping histopathologic and immunophenotypic features to serve as a diagnostic challenge.

CASE REPORT

We reported a case of a spindle cell neoplasm in an infant, which was initially considered a vascular anomaly clinically and an eventual biopsy revealed marked inflammation with a spindle cell component that was resolved as an infantile fibrosarcoma with an ETV6 break-apart.

CONCLUSION

The context of this case lead to a further consideration of various other spindle cell neoplasms arising predominantly in the soft tissues during the infancy period as defined by the first two years of age. Though sharing similar morphologic features, these tumors can be categorized into several molecular genetic groups, which have provided both diagnostic and pathogenetic insights as well as treatment options in some cases.